1. Field of the Invention
This invention relates to a ferrule of an optical fiber connector or optical connector to be used for connecting optical fibers and a method for the production thereof.
2. Description of the Prior Art
Generally, the optical fiber connector, as illustrated in FIG. 1, for example, is composed of a plug 100 to which is connected an optical fiber cord 110 having an optical fiber inserted and fixed therein along the central axis thereof and a hollow cylindrical sleeve 120 adapted to couple and align two such plugs. Unlike the electric connector, the optical fiber connector is particularly required to align exactly the opposed ends of two optical fibers to be connected.
For this purpose, a ferrule 101 which is made of a ceramic substance and adapted for the insertion of the leading ends of two very thin optical fibers to be connected is popularly used. The connection of the two optical fibers is attained by abutting two such ferrules 101 against each other. Specifically, it is based on the procedure which comprises causing two ferrules 101 each having the leading end of an optical fiber inserted and fixed therein to be severally fixed concentrically to two plugs 100 finished in a prescribed outside diameter, inserting the two plugs 100 into one sleeve 120 through the opposite ends thereof, and abutting the ferrules 101 against each other thereby aligning the axes of the optical fibers.
The conventional ferrules for optical fiber connectors have been known to come in such a typical shape as is disclosed, for example, in Japanese Patent Publication No. 1-45042. The ferrule of this shape comprises a ferrule or capillary 101 having formed along the axial line thereof a through-hole 102 having a small diameter and an ample length for the insertion of an optical fiber (or an optical fiber strand) and a flange 104 fixed to one end part of the ferrule 101 and having a through-hole 105 of a large diameter formed along the axial line thereof for the insertion of a sheathed optical fiber (an optical fiber covered with a sheath) as illustrated in FIG. 2. It is assembled by causing the end part of the ferrule 101 having a tapered bore part 103 formed therein to be fixed by close fit or adhesion to a leading end hole part 106 of the flange 104 and consequently enabling the through-hole 102 of a small diameter in the ferrule 101 to be connected to the through-hole 105 of a larger diameter in the flange 104 through the medium of the tapered bore part 103.
The attachment of the optical fiber to the ferrule 101 of this construction is implemented by stripping a sheathed optical fiber 107 of a sheath 109 at the leading end part thereof thereby exposing an optical fiber 108 in a prescribed length, as illustrated in FIG. 3, applying an adhesive agent to the exposed optical fiber and the leading end part of the sheathed optical fibers then inserting the exposed optical fiber 108 into the through-hole 102 of a small diameter of the ferrule 101 as shown in FIG. 2 from the flange side, and fixing the leading ends of the optical fiber 108 and the sheathed optical fiber 107 with the adhesive agent to the interiors of the through-hole 102 of the ferrule 101 and the through-hole 105 of the flange 104.
Besides the ferrule described above, a ferrule in which a through-hole containing both a part of a small diameter (hereinafter referred to briefly as "small diameter part") for the insertion of an optical fiber and a part of a large diameter (hereinafter referred to briefly as "large diameter part") for the insertion of the sheathed optical fiber is formed along the axial line thereof has been known as disclosed in published Japanese Patent Application, KOKAI (Early Publication) No. 7-174937, for example.
Quite naturally, these ferrules are invariably required to manifest high working accuracy in their outside diameters and in the inside diameters of their holes of a small diameter for the insertion of an optical fiber. Further, the degrees of eccentricity and parallelism of the outside diameters and the inside diameters mentioned above relative to the relevant axial lines constitute themselves important conditions for the accuracy of connection of the two optical fibers.
By this reason, the ferrules mentioned above are produced by extrusion molding or injection molding a ceramic substance thereby forming a ceramic ferrule blank containing a through-hole (formed hole), calcining the ceramic ferrule blank, passing a wire through the through-hole in the calcined ceramic ferrule blank, lapping the through-hole with diamond paste thereby imparting a prescribed inside diameter to the ferrule blank, passing a wire through the finished through-holes of a multiplicity of ferrule blanks, and performing a grinding work on the multiplicity of ferrules simultaneously thereby imparting a prescribed outside diameter thereto.
This method indeed has the advantage of allowing the grinding work for the impartation of an outside diameter to be performed simultaneously on a plurality of ferrule blanks and nevertheless incurs the disadvantage of spending a rather greater working time in the light of the necessity for allowing ample finishing allowance (grinding allowance) owing to the accumulation of dimensional variation among the individual ferrules due to the shrinkage after calcining. This method, when the ferrule itself has a through-hole of such a construction that the small diameter part for the insertion of an optical fiber and the large diameter part for the insertion of a sheathed optical fiber are connected to each other through the medium of a tapered part, is at a disadvantage in securing good concentricity between the outer circumference and the small diameter part of the through-hole only with difficulty because the length of the small diameter part for the insertion of an optical fiber as the basis for alignment is small.
Methods for overcoming this disadvantage have been proposed which are based on a common procedure comprising the steps of imparting an inside diameter to each of the ferrule blanks and then performing a grinding work for the impartation of an outside diameter with a cylindrical grinder on one after another of the ferrule blanks having a finished through-hole, as disclosed in published Japanese Patent Applications, KOKAI No. 5-113522 and No. 6-208042, for example. The methods thus proposed invariably adopt a mechanism which comprises holding one end of a blank by a work retaining part of a rotationally driven lathe dog or carrier and rotating the blank by transmitting the rotation of the lathe dog to the blank through the medium of the work retaining part. To be cylindrically ground, therefore, the ferrule must be provided with a leg portion to be held. Since this leg portion must be cut off the ferrule after the grinding work for the impartation of an outside diameter has been completed, this cutting work forms an extra burden. Since the lathe dog is utilized for the rotation of the blank, the preparatory steps for the attachment and detachment of the lathe dog inevitably bring about an increase in the time required for the grinding work even when the grinding work for the impartation of an outside diameter is automated. Thus, the number of component steps of the work increases, the time required for the work elongates, and the yield lowers. Accordingly, the methods are at a disadvantage in suffering these factors to increase notably the working cost.
When the ceramic blank of such a construction as possesses a through-hole containing the small diameter part for the insertion of an optical fiber and the large diameter part for the insertion of a sheathed optical fiber is to be obtained by a procedure involving the step of calcining subsequently to the step of injection molding, the formation of the small diameter part for the insertion of an optical fiber will inevitably require the use of a core pin containing a part of a very minute diameter as an injection molding die. This operation, therefore, has the problem of exposing the core pin to the possibility of sustaining breakage or shortening the service life because the core pin is bent by the injection pressure during the injection molding or suffered to generate bending stress each time the injection molding is repeated. From the viewpoint of solving this problem, an idea may be conceived of shortening the length of the part of the core pin having a minute diameter and consequently the length of the small diameter part in the ceramic blank for the insertion of an optical fiber. When the aforementioned method of first passing a wire through the through-hole of the ceramic blank and then grinding the ceramic blank for the impartation of an outer diameter is adopted for implementing this idea, however, it has the problem of impairing the concentricity between the outer circumference and the small diameter part of the through-hole and the cylindricality (deviation from a cylindrical form). Thus, the decrease in the length has its own limit. Even by the method which utilizes the rotary drive of the lathe dog mentioned above for the grinding work aimed at imparting an outside diameter, therefore, it is held that the shortest total length of the small diameter part for the insertion of an optical fiber and the tapered part that is attainable at all is not less than 6 mm.
Further, the core pin of the construction mentioned is difficult to fabricate and consequently is extremely expensive. The bend or the breakage which this core pin incurs, therefore, forms a large factor for boosting the cost of the ferrule.
In the case of the ferrule manufactured by the conventional method and having a long small-diameter through-hole for the insertion of an optical fiber, the breakage inflicted to the leading end part of the optical fiber during the insertion of the optical fiber into the ferrule or during the use of the optical fiber connector often poses a problem. Specifically, since the optical fiber is a very thin glass fiber, it has the drawback of readily sustaining a breakage. The insertion of the conventional optical fiber into a ferrule is attained by stripping the leading end part of the sheathed optical fiber 107 of the sheath 109 over a length greater than the length of the through-hole of a small diameter in the ferrule for the insertion of an optical fiber as illustrated in FIG. 3 and inserting the exposed part of the sheathed optical fiber 107 into the ferrule from the flange 104 side as illustrated in FIG. 2, for example. When the leading end of the exposed optical fiber collides against the inner surface of the tapered hole part 103 of the ferrule 101, bending stress (or buckling stress) is exerted on the leading end of the optical fiber. At this time, the stress is concentrated at the joint A between the sheathed optical fiber 107 and the exposed optical fiber 108 and the optical fiber 108 consequently becomes liable to bend at the joint. The breakage of the optical fiber of this nature can occur under the impact which is possibly generated when the optical fiber connector is accidentally dropped. Also for the sake of preventing the optical fiber from accidentally sustaining breakage during the work of inserting the optical fiber into the ferrule or during the use of the optical fiber connector, the merit of shortening the length of the small diameter part in the ferrule for the insertion of an optical fiber thereby allowing a decrease in the length of the exposed optical fiber has been finding growing recognition.